1. Field of the Invention
The present invention relates to silicon nitride sintered bodies which have superior mechanical properties in a range of low and intermediate temperatures ranging from room temperature to 1100.degree. C. and are useful as structural ceramic materials for automobile parts, wear resistant tools or the like. The present invention also relates to a process for producing such silicon nitride sintered bodies.
2. Description of the Prior Art
Silicon nitride is a material well balanced in strength, fracture toughness, corrosion resistance, wear resistance, thermal shock resistance and oxidation resistance, etc., and has been extensively used in a wide variety of applications, such as cutting tools, engine parts or the like. Especially, in recent years, sintered bodies of silicon nitride have attracted attention as structural materials for automobile engines, gas turbines, etc. However, in order to use the sintered bodies of silicon nitride in fields requiring high reliable materials, for example, in automobile engines, etc., it is indispensable to further improve the fracture toughness of the silicon nitride bodies to overcome the brittleness thereof and, at the same time, improve the strength.
For such requirements, for example, as disclosed in Japanese Patent Application Laid-Open No. 62-265173, there has been proposed a technique for dispersing silicon carbide whiskers in a silicon nitride matrix. It is thought that according to this technique, the fracture toughness is improved by that cracks, which may progress and expand during the fracture, are deflected by the whiskers or extraction or crosslinking of the whiskers takes place. However, since it is substantially difficult to completely remove the agglomerates of the whiskers by a mechanical means and the size thereof is of the order of 1 to 10 .mu.m, such whiskers or agglomerates act as fracture origins of the sintered body, thereby lowering the strength thereof.
Further, for example, as disclosed in Japanese Patent Application Laid-Open No. 63-159256, there has been proposed a silicon nitride-silicon carbide composite sintered body in which silicon carbide having an average particle size of 1 .mu.m or less is evenly dispersed in silicon nitride. However, even with the aforementioned composite sintered body of silicon nitride-silicon carbide composite sintered body, an insufficient proportion of silicon carbide tends to form columnar crystal grains of silicon nitride and causes the abnormal grain growth. Therefore, the defect size is increased and only a very slight improvement is observed in the strength although some improvement is obtained in the fracture toughness. On the other hand, an excessive proportion of silicon carbide suppresses the formation of columnar crystal grains of silicon nitride, thereby lowering the fracture characteristics contrary to an increase in strength.
As set forth above, it has been extremely difficult to simultaneously improve the strength and fracture toughness since, in known attempts as described above to increase the strength or fracture toughness of silicon nitride sintered bodies, an increase in the strength by refinement of the structure lowers the fracture toughness and conversely, an improvement in the fracture toughness by the addition of whiskers or by allowing large columnar crystals to exist by the grain growth of silicon nitride leads to the lowering of the strength.